A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacturing of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. including part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the beam of radiation in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
Photolithography is widely recognized as one of the key steps in the manufacturing of ICs and other devices and products with small features. However, as the dimensions of features become smaller, photolithography is becoming one of the most, if not the most, critical gating factors for enabling ICs and other devices and products with small features to be manufactured on a massive scale.
A theoretical estimate of the limits of feature printing can be given by the Rayleigh criterion for resolution R as shown in equation (1):
                    R        =                              k            1                    *                      λ            NA                                              (        1        )            where λ is the wavelength of the radiation used, NA is the numerical aperture of the projection system used to image the feature, and k1 is a process dependent adjustment factor, also called the Rayleigh constant.
It follows from equation (1) that the resolution of any given feature can be improved in three ways: by shortening the exposure wavelength λ, by increasing the numerical aperture NA, or by decreasing the value of k1. All of these strategies have been pursued simultaneously in the past and are expected to continue in the future. For conventional optical lithography, the ultimate resolution limit is reached at k1=0.5, which corresponds to the state at which only one set of diffracted orders can pass through the projection system. The resolution limit of k1=0.5 stands firm even as exposure wavelengths decrease from 248 nm to 193 nm and 157 nm, and numerical aperture increases from 0.5 to 0.75.
One solution that was recently proposed to print complex patterns with sufficient latitude for manufacturing ICs at minimum half pitches of k1=0.5 or below, is to use a vortex mask. (See Mark D. Levenson et al., “The Vortex Mask: Making 80 nm Contacts with a Twist!”, 22nd Annual BACUS Symposium on Photomask Technology, Proceeding of SPIE Vol. 4889 (2002)). A vortex mask is composed of rectangles with phases of 0 degrees, 90 degrees, 180 degrees and 270 degrees. The walls of the phase trenches are nearly vertical, with all four-phase regions meeting at sharp corners, which define the phase singularities. Because the phase of the wave front is not defined at the corner where the rectangles with the four different phases meet, the intensity at that point is zero. In other words, the central core of the vortex is dark. Therefore, after traversing the vortex mask, the radiation wavefront spirals like a vortex and has a zero amplitude on its central core, instead of forming a plane or a sphere. In combination with a negative resist process, the central axis dark spot of the optical vortex transferred onto the substrate may potentially support larger process windows at small k1 (based on half pitch) than conventional methods and may allow for smaller features to be printed with acceptable process latitude. However, a successful implementation of this technology requires the development of appropriate negative-resist tone processes which may be complicated and costly.